• Konstantin Khlebnikov's avatar
    trylock_super(): replacement for grab_super_passive() · eb6ef3df
    Konstantin Khlebnikov authored
    I've noticed significant locking contention in memory reclaimer around
    sb_lock inside grab_super_passive(). Grab_super_passive() is called from
    two places: in icache/dcache shrinkers (function super_cache_scan) and
    from writeback (function __writeback_inodes_wb). Both are required for
    progress in memory allocator.
    Grab_super_passive() acquires sb_lock to increment sb->s_count and check
    sb->s_instances. It seems sb->s_umount locked for read is enough here:
    super-block deactivation always runs under sb->s_umount locked for write.
    Protecting super-block itself isn't a problem: in super_cache_scan() sb
    is protected by shrinker_rwsem: it cannot be freed if its slab shrinkers
    are still active. Inside writeback super-block comes from inode from bdi
    writeback list under wb->list_lock.
    This patch removes locking sb_lock and checks s_instances under s_umount:
    generic_shutdown_super() unlinks it under sb->s_umount locked for write.
    New variant is called trylock_super() and since it only locks semaphore,
    callers must call up_read(&sb->s_umount) instead of drop_super(sb) when
    they're done.
    Signed-off-by: default avatarKonstantin Khlebnikov <khlebnikov@yandex-team.ru>
    Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
super.c 35.3 KB